Study to Assess Efficacy and Safety of Baloxavir Marboxil In Combination With Standard-of-Care Neuraminidase Inhibitor In Hospitalized Participants With Severe Influenza

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This study will evaluate the efficacy, safety, and pharmacokinetics of baloxavir marboxil in combination with a standard-of-care (SOC) neuraminidase inhibitor (NAI) (i.e., oseltamivir, zanamivir, or peramivir) compared with a matching placebo in combination with a SOC NAI in hospitalized patients with influenza.

Participants will receive at least two doses of baloxavir marboxil or its matching placebo on Day 1 and 4. A third dose of Baloxavir or its matching placebo will be given on Day 7 for participants who have not improved according to protocol defined criteria on Day 5

Study treatment will be given in combination with SOC NAI (i.e., oseltamivir, zanamivir, or peramivir) in accordance with local clinical practice. SOC NAI will be administered to cover a minimum of treatment exposure from Day 1 to Day 5

Drug: Baloxavir Marboxil

Baloxavir marboxil will be administered as a weight-based dose on Days 1 and 4. A third dose of Baloxavir or its matching placebo will be given on Day 7 for participants who have not improved according to protocol defined criteria on Day 5

Treatment with a SOC NAI (i.e., oseltamivir, zanamivir, or peramivir) will also be administered in accordance with local clinical practice.

Placebo Comparator: Placebo

Participants will be assigned in a 2:1 ratio to receive baloxavir marboxil or matching placebo.

Study treatment will be given in combination with SOC NAI (i.e., oseltamivir, zanamivir, or peramivir) in accordance with local clinical practice. SOC NAI will be administered to cover a minimum of treatment exposure from Day 1 to Day 5

Other: Placebo

Participants will receive Baloxavir marboxil or matching placebo in 2:1 ratio. Treatment with a SOC NAI (i.e., oseltamivir, zanamivir, or peramivir) will also be administered in accordance with local clinical practice.

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Ages Eligible for Study:

12 Years and older (Child, Adult, Older Adult)

Sexes Eligible for Study:

All

Accepts Healthy Volunteers:

No

Criteria

Inclusion Criteria:

Adult participants: Signed informed consent by any participant capable of giving consent, or, where the participant is not capable of giving consent, by his or her legal/authorized representative

Adolescent participants not able to legally consent: written informed consent for study participation is obtained from participant's parents or legal guardian, with assent as appropriate by the participant, depending on the participant's level of understanding and capability to provide assent

Participants who require hospitalization for severe influenza or acquire influenza during hospitalization, the severity of which requires an extension of hospitalization

For women of childbearing potential: Agreement to remain abstinent or use contraceptive methods with a failure rate of < 1% per year during the treatment period and for 28 days after the last dose of study treatment. Hormonal contraceptive methods must be supplemented by a barrier method.

Exclusion Criteria:

Participants who have received more than 48 hours of antiviral treatment for influenza prior to screening

Participants who have received baloxavir marboxil for the current influenza infection

Known contraindication to neuraminidase inhibitors

Participants hospitalized for exclusively social reasons (e.g., lack of caregivers at home)

Participants expected to die or be discharged within 48 hours, according to the investigator's judgement